I finished the main quest line in Oblivion last night. This is without a doubt the buggiest game I have ever stuck with this long, winning that race over Fallout 3 by a nose. (Bethesda nails the love-hate relationship concept.) It wasn't on my very short list above, but that's because I had forgotten all about it, or rather how much of it was unfinished.
I found there may be a way to fix the broken Shivering Isles quest line by transferring a 360 file to PC, giving myself the forever missing felldew item in the PC version's console, then transferring it back. I'm debating whether to go through the chore of assembling all the pieces. It needs an up-to-date PC version of the game + DLCs I have, and some hacking tools I'm scarcely familiar with. Maybe I'll just start a new character and head for SI ASAP. Don't know yet. I do love my uber-powerful fighter/mage/invisible sneaky guy in perfect madness armor.